SecondaryCommandBuffer is a CPU-side command buffer construct intended
to delay allocation/construction of GPU-side command buffers until
absolutely necessary.

Initially ANGLE was batching commands into Vulkan secondary command
buffers and then submitting those command buffers when rendering was
required. On at least some devices we saw two areas of overhead that
SecondaryCommandBuffers are intended to reduce:
1. Commands in secondary cmd buffers taking longer than equivalent
commands in a single primary cmd buffer.
2. Allocation/free/reset overhead of the secondary command buffers was
a hotspot for some workloads.

Bug: angleproject:3136
Change-Id: Ife8ffe2968eee423d89ff433d62596c432156661
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1492016
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Jamie Madill <jmadill@google.com>